This invention relates to telephoto lenses, and more particularly to a highly corrected Ernostar type lens.
Recently, from the standpoint of easy management with less stress on photographic performance, there has been a demand for compactness of photographic lenses. This demand is particularly acute in the field of intermediate telephoto lenses of focal lengths on the order of 100 to 150 m/m, because they are suited for many photographic applications such as portrait photography.
The Ernostar lens is prominent around the types of telephoto lenses adapted to have focal lengths within this range. An Ernostar type lens provides for compactness with an increase in relative aperture. However, difficult problems arise in that the zonal spherical aberration tends to increase along with the Petzval sum in the negative sense, and that the spherical aberration for light rays of shorter wave lengths than the design wave length is extremely over-corrected. This results in production of some flare in the picture image.
To reduce the physical length of a five component Ernostar type telephoto lens, the refractive power of the first component counting from the front is usually increased, the axial distance between the front surface of the first component and the rear surface of the third component is elongated, and the refractive power of the third component is increased. As has been stated above, however, this results in an unavoidable increase in the zonal spherical aberration as the refractive power of the first component increases, and an increase in the Petzval sum in the negative sense as the above-defined distance increases. In addition thereto, since the third component has a large chromatic dispersion for the purpose of chromatic correction, the increase in the absolute refractive power of the negative third component leads to a large increase in over-correction of spherical aberration for the shorter wave length light rays.
Japanese Patent Publication No. Sho 49-23771 issued 1974 to the assignee of the present invention discloses an example of an Ernostar type telephoto lens designed to have a telephoto effect of 0.98, that is the length of the entire system from front vertex to film plane which is equal to 0.98 times the focal length of the system, with a F-number of 1:2.5 and an image angle of 18.2.degree.. It is now found that the lens described in this patent leaves room for improvements of further shortening of the physical length and of correction of spherical aberration. Another example of a recently developed Ernostar type lens is described in U.S. Pat. No. 4,057,330.